At present, a general and the most efficient antenna element is a dipole antenna of a half wave length of the frequency of the signal used for receiving or transmitting. However, a half-wave length dipole antenna for a frequency band below the VHF band is considerably large in size from the viewpoint of practical use.
Such a large-sized antenna is difficult to install and has many problems in maintenance and safety. Therefore, from a viewpoint of further expanding the flexibility in the installation of the antenna, it is also desirable to make the antenna small-sized.
Hitherto, it has been tried or proposed in other fields to make the dipole antenna small-sized, in which antenna gain has been very low in comparison with the half-wave length dipole antenna.
The reason for this is that loading means, which is a technical requirement for contracting the dipole antenna in length, results in a very large loss in radiation power caused by the resistance loss existing in a small-sized dipole antenna which embodies the loading means, thereby considerably reducing the radiation power so as to result in a low radiation efficiency.
Lowering of radiation efficiency, of course, means the lowering of the antenna gain.
Furthermore, even with the aforesaid half-wave length dipole antenna, set values of V.S.W.R. at feed terminals cannot be ideal at all frequency points within the corresponding frequency band, which is due to the fact that the half-wave length dipole antenna of a fixed length has only one frequency resulting in a perfect resonance condition, and has an essential defect that the values at V.S.W.R. are of course deteriorated at other frequencies. The defect causes mismatching in the characteristic impedance between the half-wave dipole antenna and the radio receiver connected thereto. A reflection loss exists in reception signal energy transmission due to the mismatching, thereby lowering the gain of the whole antenna system including the antenna and radio receiver.
From the above, a small-sized antenna is required which has a high gain and ideal values of V.S.W.R. at the characteristic impedance of feed terminals at all frequency points within a corresponding frequency band.